The field of the present invention is digital signal processing systems and, more particularly, decision feedback equalizer circuits for use in such systems.
Recently, substantial attention has been directed toward the development of local and wide area networks and communications systems that use multilevel pulse signaling schemes. These networks generally employ receiver circuits having a baseband equalization capability. Stated somewhat differently, for receiver circuits to consistently decipher high, low and intermediate pulses representing data symbols, it is necessary for those circuits to equalize or compensate for transmission loss and phase distortion within the baseband signal. One of the most common forms of noise that must be removed from received baseband signals is generally referred to as intersymbol interference (ISI) and results from the variable low pass characteristics of the transmission media (generally, a twisted pair of cable).
As shown in FIG. 1, a baseband square wave pulse that is passed through a low pass filter or low pass transmission network may be substantially altered, disfigured and/or re-shaped due to the resistive, capacitive and inductive nature of a low pass transmission system. In short, the generated baseband square wave is transformed into a signal that rises to a peak value during the period T of the square wave and then gradually decreases or "tails off" to some reduced level in the period following the falling edge of the baseband square wave signal. Not surprisingly, the tail portion of the transformed signal may cause substantial interference or noise to be found in later received signals. Indeed, the tail portion of the transformed signal is one of the principal causes of post cursor intersymbol interference (ISI). Digital decision feedback equalizer circuits have been known to compensate for such post cursor intersymbol interference, and have proven to be an integral part of modern broad band communication receivers. However, despite the fact that digital decision feedback equalizer circuits have been used extensively in modems and other forms of broadband receivers, their use in very high frequency or high speed baseband multilevel receivers has been extremely sparse. This is primarily due to the fact that conventional digital signal processing techniques rely on at least a two times (2.times.) oversampling of analog baseband waveforms to gather sufficient quantized information to perform equalization in the digital domain. This, however, becomes an extremely complex and, indeed, prohibitive process when the baseband signaling rate calls for pulse widths in the range of 10 nanoseconds and below. Stated somewhat differently, there is simply too little time available to perform digital decision feedback equalization functions where the baseband signaling rate calls for pulse widths in the range of 10 nanoseconds and below.
Accordingly, it is respectfully submitted that those skilled in the art would find a decision feedback equalizer circuit capable of functioning at baseband frequencies wherein the pulse widths of the transmitted signals are less than or equal to 10 nanoseconds to be quite useful.